Inundation Gradient Research Articles

Coastal inundation regime moderates the short-term effects of sediment and soil additions on seawater oxygen and greenhouse gas dynamics: a microcosm experiment

The frequency and persistence of tidal inundation varies along the coastal terrestrial-aquatic interface, from frequently inundated wetlands to rarely inundated upland forests. This inundation gradient controls soil and sediment biogeochemistry and influence the exchange of soils and sediments from terrestrial to aquatic domains. Although a rich literature exist on studies of the influence of tidal waters on the biogeochemistry of coastal ecosystem soils, few studies have experimentally addressed the reverse question: How do soils (or sediments) from different coastal ecosystems influence the biogeochemistry of the tidal waters that inundate them? To better understand initial responses of coastal waters that flood coastal wetlands and uplands, we conducted short-term laboratory experiments where seawater was amended with sediments and soils collected across regional gradients of inundation exposure (i.e., frequently to rarely inundated) for 14 sites across the Mid-Atlantic, USA. Measured changes in dissolved oxygen and greenhouse gas concentrations were used to calculate gas consumption or production rates occurring during seawater exposure to terrestrial materials. We also measured soil and water physical and chemical properties to explore potential drivers. We observed higher oxygen consumption rates for seawater incubated with soils/sediments from frequently inundated locations and higher carbon dioxide production for seawater incubated with soils from rarely inundated transect locations. Incubations with soil from rarely inundated sites produced the highest global warming potential, primarily driven by carbon dioxide and secondarily by nitrous oxide. We also found environmental drivers of gas rates varied notably between transect locations. Our findings indicate that seawater responses to soil and sediment inputs across coastal terrestrial-aquatic interfaces exhibit some consistent patterns and high intra- and inter-site variability, suggesting potential biogeochemical feedback loops as inundation regimes shift inland.

Changes in Soil Organic Carbon Content Under Different Inundation Gradients in Peat Bogs on the China's Qinghai-Tibet Plateau

Changes in Soil Organic Carbon Content Under Different Inundation Gradients in Peat Bogs on the China's Qinghai-Tibet Plateau

Distinguishing the mechanisms driving multifaceted plant diversity in subtropical reservoir riparian zones.

Understanding the multifaceted plant diversity and its maintenance mechanisms is crucial for biodiversity conservation. Dam-induced water level fluctuations dramatically alter various aspects of riparian diversity, such as taxonomic (TD), phylogenetic (PD), or functional (FD) diversity. However, few studies simultaneously evaluated plant TD, FD, and PD, especially in the subtropical reservoir riparian zone. Here we sampled plant diversity and environmental drivers along inundation gradients of the Three Gorges Reservoir Region in China. We integrated multifaceted plant diversity to assess how distinct ecological processes affect the plant community assembly and how they respond to inundation gradients, spatial variability, climate, and soils in dam-regulated riparian zones. We found that alpha TD, PD, and FD diversity exhibited decreasing trends with increasing inundation gradients and significant positive correlations with soil organic matter. The number of clustering plant communities increases along the inundation gradients. Beta TD and PD diversity were mainly dominated by species turnover with fewer contributions from nestedness, while beta FD diversity was mainly dominated by nestedness with fewer contributions from species turnover. The explainable rates of different dimensions of beta diversity, turnover, and nestedness ranged from 11% to 61%, with spatial factors explaining the highest beta diversity in different dimensions, followed by inundation gradients, soil properties, and climate variables. Our results suggest dispersal limitations are more important for species turnover in dam-regulated riparian zones at regional scales, while inundation gradients and soil fertility are more critical in shaping plant community assemblages at the local scale. This study emphasizes that environmental and spatial gradients are critical for understanding the assembly mechanisms driving multifaceted plant communities at local and regional scales and reinforces the importance of protecting seed sources and dispersal pathways and maintaining river connectivity when implementing restoration projects.

Plant communities regulated by water-level gradient in Caohai aquatic–terrestrial ecotones affect bacterial and fungal structures and co-occurrence networks

There is increasing awareness that vegetation dynamics play an important role in ecosystem stability and belowground microbial community diversity in wetlands. Studying plant–microbe dynamics and interactions under various conditions within lakes helps with the effective management of lake ecosystems by restoring ecosystem services. However, our knowledge about the response of microbial interactions to vegetation variation remains limited. We investigated how soil nutrients, enzyme activities, fungal and bacterial community diversity, and their network properties vary along the inundation gradient in aquatic–terrestrial ecotones of Caohai Lake in southwest China. Compared with the arid zone, the organic carbon and ammonia nitrogen contents in the severe inundation zone (SIZ) increased by 28.5% and 16.4%, respectively. Compared with the SIZ, the activities of cellobiohydrolase, β-1, 4-N-acetylglucosaminidase and leucine aminopeptidase in the dry–wet cycling zone increased significantly, increasing by 62.1%, 60.5% and 26.1%, respectively. Moreover, long inundation significantly reduced the abundance of fungi, diversity of bacteria, and size and complexity of their community co-occurrence networks. Microbial networks were smaller (fewer edges) and less stable (higher positive to negative edge ratios) in the SIZ. Aboveground vegetation regulated by water-level gradient can directly or indirectly affect microbial community structure and further interfere with the properties of bacterial and fungal co-occurrence networks. In conclusion, our findings suggested that plant communities regulated by water-level gradient play an important role in maintaining microbial diversity and co-occurrence network stability along the aquatic–terrestrial ecotone in Caohai Lake.

Dry-Wet Cycles Affect Nitrous Oxide Emissions Across Aquatic-Terrestrial Interfaces: A Mesocosms Study

Aquatic-terrestrial interfaces may act as biogeochemical hotspots for greenhouse gas emissions, especially when exposed to frequent transitions between wet and dry phases. The study aimed to analyze the dynamics of nitrogen (N) processing along an inundation gradient from floodplain soils to river sediments and identify environmental factors affecting net nitrous oxide (N2O) production from different microbial sources. Intact soil and sediment cores were subject to two consecutive drying-rewetting cycles in laboratory experiments. The 15N isotope pairing technique was used to quantify N2O emissions sourced from denitrification and nitrification. We observed enhanced N2O emissions from both nitrification and denitrification following drying events. Sites exposed to frequent drying-rewetting cycles appear less affected by drying than hydrologically more stable habitats. Fluxes from nitrification were related to the organic matter content, while fluxes from denitrification were controlled by dissolved organic matter quality changes during the drying-rewetting cycles. This study shows the potential link between carbon metabolism and N2O production, combining the effect of drying-rewetting cycles.

Traits of tidal marsh plants determine survival and growth response to hydrodynamic forcing: implications for nature-based shoreline protection

Tidal marshes are increasingly valued for their nature-based shoreline protection function, as they reduce waves, currents and erosion. The effectiveness of this function depends on the ability of tidal marsh plants to grow and survive under pressure from waves and currents. However, how this varies with species-dependent plant traits is poorly understood. We performed a field transplantation experiment to quantify species-specific growth responses to different levels of hydrodynamic exposure and tidal inundation for 3 NW European marsh species: Schoenoplectus tabernaemontani, Bolboschoenus maritimus and Phragmites australis. In this order, these species showed increasing shoot stiffness, length and biomass, which are traits that increase hydrodynamic drag forces experienced by plants. Increased exposure to tidal inundation and hydrodynamics reduced the growth of all 3 species, but species with lower biomass and shorter, thinner and more flexible shoots could better cope with higher hydrodynamic exposure and tidal inundation. Furthermore, transplants of S. tabernaemontani (i.e. the species with the lowest shoot stiffness, length and biomass that survived under all tested conditions) developed smaller, thinner and more flexible shoots in response to higher hydrodynamic exposure and inundation. Hence our study indicates that similar inter- and intra-specific plant traits drive plant growth in response to hydrodynamics and inundation. This suggests that the spatial distribution of species typically observed in tidal marshes results not only from species-specific tolerance to tidal inundation gradients but also from hydrodynamic gradients. Allowing enough space for development of species zonation may be important to increase the efficiency of nature-based shoreline protection by tidal marshes.

Dam inundation simplifies the plant community composition

The construction of dams has caused riparian habitat degradation and ecosystem service loss globally. It is critical to assess the response of riparian plant communities to inundation gradients for their conservation. Recent evidence suggests that plant community assemblages are governed by flooding stress, soil nutrient availability, climate (environmental filtering) and dispersal, speciation, local extinction (dispersal filtering), but it remains unclear which dominates the riparian ecosystem regulated by a dam. Thus, this article aims to elucidate the relative importance of environmental and dispersal filtering to variations in plant communities to understand community assembly mechanisms in riparian ecosystems. Here we used plant community data related to four elevations in the riparian zone of the Three Gorges Dam Reservoir in China to show that species richness and diversity, community height, and the cover of total, annual, and exotic plant categories decreased, while the cover of perennial and native plant groups increased under higher flooding stress. Community composition varied substantially with elevation, and species composition tended to converge with increased inundation, characterized by flood-tolerant species. The community composition underwent stronger environmental filtering at low elevations and stronger dispersal filtering at high elevations, with stronger environmental filtering across riparian ecosystems. Therefore, we conclude that dam inundation drives community assemblages of riparian plants by the combined effects of environmental and dispersal filtering. Still, their relative contribution varies between elevations, and environmental filtering is more important in shaping community assembly. This study is the first to confirm that plant community assembly in the dam-regulated riparian area is determined by both niche-based and stochastic processes. Thus, we highlighted the importance of considering inundation intensity, propagule sources, and river connectivity when implementing restoration projects.

Modelling mangrove forest structure and species composition over tidal inundation gradients: The feedback between plant water use and porewater salinity in an arid mangrove ecosystem

Conceptual models have described mangrove forest structure in response to tidal inundation, porewater salinity and mangrove water use, however, the importance of feedbacks have not been assessed. We assessed the processes underlying the feedback between water availability and plant water use in mangrove ecosystems by applying the process- and individual-based mangrove model MANGA, evaluating individual tree growth behavior, emerging forest structure, and mangrove species composition. We compared simulation data with field data collected at a field site with a monospecific mangrove forest that showed a strong gradient in tree height and biomass across the intertidal zone. While tidal inundation and the resulting porewater salinity gradient caused variation in mangrove forest structure within the MANGA model, the inclusion of tree water use in the model led to high similarity between model results and field observations. We further show the importance of the feedback between tree water use and porewater salinity for emergence of species zonation patterns. In accordance with particular site observations, we simulated the response to a broad spectrum of modelled porewater salinity inputs of an artificial forest. The modelled forest consists of two distinct species differing in their water use efficiency and salt tolerance. The modelled feedback mechanism produced a strong correlation between mangrove species zonation along the intertidal gradient and porewater salinity inputs at the system boundaries. The results of this study describe and demonstrate the absolute and relative importance of the implemented interacting processes on mangrove vegetation patterns. A better understanding of the role of tidal inundation and the feedback loops between plant water use and porewater salinity in determining mangrove forest structure will help to predict and manage the fate of mangrove ecosystems in a changing climate.

Distinct Community Assembly Processes of Abundant and Rare Soil Bacteria in Coastal Wetlands along an Inundation Gradient.

Microbial communities commonly consist of a large number of rare taxa (RT) and few abundant taxa (AT), and it is important to identify the differences of the community assembly processes between RT and AT in response to environmental changes. However, the community assembly processes governing AT and RT in coastal wetland soils along an inundation gradient remain elusive. Here, an in situ mesocosm, with continuous inundation gradients and native mangrove Kandelia obovata or exotic cordgrass Spartina alterniflora, was established to determine the patterns and driving factors of community turnover and assembly processes of AT and RT. We found that RT exhibited a remarkably lower turnover rate than AT, and the niche breadth of RT was significantly narrower than that of AT. In comparison with AT, RT presented stronger phylogenetic signals for ecological preferences across environmental gradients. Null model analyses revealed that RT were more phylogenetically clustered and primarily governed by homogeneous selection, while AT were more overdispersed and dominated by dispersal limitation. Soil water content was the most decisive factor for community turnover and assembly processes of both AT and RT. Structural equation modeling analysis showed that RT were strongly associated with K. obovata biomass rather than S. alterniflora biomass, suggesting a strong relationship between RT and the growth of mangrove K. obovata Overall, our study revealed distinct assembly processes of soil AT and RT communities in coastal wetlands, which is crucial for mechanistic understanding of the establishment and maintenance of soil microbial diversity in coastal wetlands under conditions of global environmental changes.IMPORTANCE Coastal wetlands are one of the important ecosystems that play a crucial role in the regulation of climate change. Rare taxa (RT) exist in one habitat along with abundant taxa (AT). In this study, we found that RT exhibited narrower niche breadth and stronger phylogenetic signals than AT. Null model analyses showed that RT were more phylogenetically clustered and primarily governed by homogeneous selection, while AT were more overdispersed and dominated by dispersal limitation. Revealing the differences in the community assembly processes between AT and RT in coastal wetlands is critical to understand the establishment and maintenance of soil microbial diversity in coastal wetlands with regard to environmental changes.

Palustrine forested wetland vegetation communities change across an elevation gradient, Washington State, USA.

BackgroundForested wetlands support distinct vegetation and hydrology relative to upland forests and shrub-dominated or open water wetlands. Although forested wetland plant communities comprise unique habitats, these ecosystems’ community structure is not well documented in the U.S. Pacific Northwest. Here I surveyed forested wetland vegetation to identify changes in community composition and structure across an elevation gradient that corresponds to flooding stress, asking: (1) How do forested wetland plant communities change across an elevation gradient that corresponds to flood frequency and duration? (2) At what relative elevations do different plant species occur within a wetland?MethodsI measured overstory tree basal area and structure and understory vascular plant composition in three zones: wetland buffers (WB) adjacent to the wetland, an upper wetland (UW) extent, and a lower wetland (LW) extent, surveying individual trees’ root collar elevation relative to the wetland ordinary high-water mark (OHWM). I estimated understory plant species abundance in sub-plots and surveyed these plots’ height above the OHWM. I used non-metric multidimensional scaling ordination to identify patterns in vegetation communities relative to wetland elevation, and tested for compositional differences between the WB, UW and LW zones using PERMANOVA. I calculated overstory and understory indicator species for each wetland zone using indicator species analysis.ResultsForest overstory composition changed across the elevation gradient, with broad-leaved trees occupying a distinct hydrologic niche in low-lying areas close to the OHWM. Conifer species occurred higher above the OHWM on drier microsites. Pseudotsuga menziesii (mean elevation = 0.881 m) and Tsuga heterophylla (mean elevation = 1.737 m) were overstory indicator species of the WB, while Fraxinus latifolia (mean elevation = 0.005 m) was an overstory indicator for the upper and lower wetland. Understory vegetation differed between zones and lower zones’ indicator species were generally hydrophytic species with adaptations that allow them to tolerate flooding stress at lower elevations. Average elevations above the OHWM are reported for 19 overstory trees and 61 understory plant species. By quantifying forested wetland plant species’ affinities for different habitats across an inundation gradient, this study illustrates how rarely flooded, forested WB vegetation differs from frequently flooded, LW vegetation. Because common management applications, like restoring forested wetlands and managing wetland responses to forest harvest, are both predicated upon understanding how vegetation relates to hydrology, these data on where different species might establish and persist along an inundation gradient may be useful in planning for anticipated forested wetland responses to restoration and disturbance.

Suppression of root-endogenous fungi in persistently inundated Typha roots

ABSTRACTWetland soils are defined by anoxic and reducing conditions that impose biogeochemically hostile conditions on plant roots and their endogenous fungal communities. The cosmopolitan wetland plant Typha L. mitigates root-zone anoxia efficiently, such that roots of these plants may constitute fungal habitats similar to roots in subaerially exposed soils. Alternatively, fungi may compete with plant cells for limited oxygen in inundated roots. We hypothesized that extrinsic environmental factors may reduce fungal incidence and affect fungal community structure within inundated roots as compared with those in subaerially exposed soils. We sampled roots of Typha spp. plants across inundation gradients in constructed reservoirs; root subsamples were microscopically examined for fungal structures, and morphologically distinct fungal endophytes were cultured and isolated from surface-sterilized subsamples. We found that the incidence of fungal hyphae was suppressed for all types of vegetative mycelia when roots were inundated, regardless of depth, but that there were no obvious differences in community composition of fungi cultured from roots growing in inundated versus subaerially exposed soils. This suggests that the suppression of hyphae we observed in root samples did not result from changes in community composition. Instead, low hyphal incidence in inundated Typha roots may reflect germinal inhibition or unsuccessful initial colonization, possibly owing to plant-mediated redox dynamism in the surrounding soil. No variation was seen in the incidence of asexual spores, or chytridiomycetes, nor were there significant differences between geographically disparate sampling sites. Communities of root-endogenous fungi may therefore be influenced more strongly by external environmental factors than by the environments that plant roots comprise.

Integrating UAV optical imagery and LiDAR data for assessing the spatial relationship between mangrove and inundation across a subtropical estuarine wetland

Mangrove-inundation patterns have been examined in many field studies, but spatially explicit quantitative analyses are still lacked. A better understanding of how mangrove species responds to the inundation gradient will facilitate mangrove conservation and management. In this study, we integrated unmanned aerial vehicle (UAV) camera imagery and light detection and ranging (LiDAR) data for exploring mangrove-inundation spatial patterns over the intertidal zone in a subtropical estuarine wetland (Zhangjiang estuarine wetland, Fujian, China). Our results indicated that >90% of mangrove forests (mainly Kandelia obovate, Avicennia marina, and Aegiceras corniculatum) were situated within a one-meter elevation range between local mean sea level and higher high water, and the spatial distribution of mangrove forests showed hump-shaped patterns along the inundation gradient with favorable inundation periods of 2 ∼ 5 h per day. Our analyses further indicated that there was a weak mangrove zonation pattern with overlaps between species zones along the elevation gradient, and that mangrove species had differential inundation sensitivities with Avicennia marina being most sensitive to the inundation stress. To the best of our knowledge, this is the first spatially explicit quantitative study to examine the influences of inundation regime on both spatial distribution and canopy height of different mangrove species over the intertidal zone at the landscape scale. This study highlights the practicability and necessity of UAV LiDAR in interpreting mangrove-inundation spatial patterns, and confirms the importance of inundation regime as a key driver in regulating the spatial patterns of mangrove forests. The determination of appropriate elevation ranges and inundation periods provides some important knowledge to guide mangrove restoration practices and to assess the vulnerability of mangrove forests in response to future sea level rise.

Uncertainty in United States coastal wetland greenhouse gas inventorying

Coastal wetlands store carbon dioxide (CO2) and emit CO2 and methane (CH4) making them an important part of greenhouse gas (GHG) inventorying. In the contiguous United States (CONUS), a coastal wetland inventory was recently calculated by combining maps of wetland type and change with soil, biomass, and CH4 flux data from a literature review. We assess uncertainty in this developing carbon monitoring system to quantify confidence in the inventory process itself and to prioritize future research. We provide a value-added analysis by defining types and scales of uncertainty for assumptions, burial and emissions datasets, and wetland maps, simulating 10 000 iterations of a simplified version of the inventory, and performing a sensitivity analysis. Coastal wetlands were likely a source of net-CO2-equivalent (CO2e) emissions from 2006–2011. Although stable estuarine wetlands were likely a CO2e sink, this effect was counteracted by catastrophic soil losses in the Gulf Coast, and CH4 emissions from tidal freshwater wetlands. The direction and magnitude of total CONUS CO2e flux were most sensitive to uncertainty in emissions and burial data, and assumptions about how to calculate the inventory. Critical data uncertainties included CH4 emissions for stable freshwater wetlands and carbon burial rates for all coastal wetlands. Critical assumptions included the average depth of soil affected by erosion events, the method used to convert CH4 fluxes to CO2e, and the fraction of carbon lost to the atmosphere following an erosion event. The inventory was relatively insensitive to mapping uncertainties. Future versions could be improved by collecting additional data, especially the depth affected by loss events, and by better mapping salinity and inundation gradients relevant to key GHG fluxes. Social Media : US coastal wetlands were a recent and uncertain source of greenhouse gasses because of CH4 and erosion.

Water sources of plant uptake along a salt marsh flooding gradient.

Salt marsh plants are affected by regular tidal inundation exposing them to saline water as a potential water source. This study aimed at quantifying the water uptake of plants depending on their distance from the sea and exploring plant responses to changing inundation regimes. We used stable isotope ratios (δ18O) to determine the proportions of seawater and precipitation water used by three salt marsh species (Spartina anglica, Atriplex portulacoides and Elytrigia atherica) from a German North Sea coast salt marsh. Additionally, A. portulacoides was transplanted to experimental islands at three elevation levels to investigate its plasticity in water use in the course of future sea level rise. We found a marked gradient in plant seawater use from the lowermost pioneer zone (79-98% seawater uptake by S. anglica) to the lower marsh (61-95% by A. portulacoides) and the upper marsh (25-39% by E. atherica). Seasonal differences in water use were not pronounced, likely due to the absence of longer dry periods during summer in these temperate salt marshes. Contradicting our expectation, roots in deeper soil showed higher water uptake rates per fine root mass than topsoil roots suggesting effective root adaptation to the anoxic subsoil. Transplanted A. portulacoides plants significantly increased the uptake of seawater with increasing inundation indicating flexibility in the use of water sources by this species which may facilitate acclimation to rising sea levels. We conclude that the zonation of salt marsh vegetation reflects the availability of water sources along the inundation gradient.

Temporal and spatial variation of Myriapoda (Diplopoda and Chilopoda) assemblages in a Neotropical floodplain

Abstract: Myriapods constitute important edaphic macrofauna taxa which dwell in different trophic levels and influence the dynamics of these environments. This study evaluated the variation in composition, richness and abundance of edaphic myriapod assemblages as a function of the distribution and structure of flooded and non-flooded habitats (spatial variation) and hydrological seasonality (temporal variation) in a floodplain of the northern Pantanal region of Mato Grosso, Brazil. Sampling was carried out in three areas of the Poconé Pantanal, along an altitudinal and inundation gradient consisting of inundated and non-inundated habitats and different vegetation formations. Three quadrats (10 x 10 m) were delimited within each habitat type, where sampling was performed using pitfall traps and mini-Winkler extractors during the dry, rising water, high water and receding water periods of two hydrological cycles within the Pantanal (2010/2011 and 2011/2012). A total of 549 millipedes were collected, consisting of 407 Diplopoda and 142 Chilopoda distributed in six orders, 12 families and 20 species. The assemblages composition varied throughout the seasonal periods, indicating that the rising water and dry periods differed from the high water and receding water periods. In addition to the variation between seasonal periods, myriapod richness and abundance also varied in relation to areas consisting of different vegetation formations. Thus, it can be concluded that the hydrological seasonality associated with the inundation gradient and different vegetation types were determinant in the heterogeneous spatial and temporal distribution of myriapod assemblages, validating that the conservation of these invertebrates in the Pantanal is directly linked to the preservation of vegetation, and consequently, ecosystem integrity.

Reflection of stable isotopes and selected elements with the inundation gradient at the Matang Mangrove Forest Reserve (MMFR), Malaysia

Multi-elemental and isotopic fingerprints could help to track changes in mangrove forest productivity and development. This study aims to identify elemental concentrations and stable isotope ratios in inter- and intra-mangrove species along the inundation gradient of the Matang mangrove forest. Matured sun leaves of different mangrove species, namely, Avicennia alba, Rhizophora apiculata, Bruguiera parviflora, and Sonneratia ovata, were collected from the Matang Mangrove Forest Reserve (MMFR). After samples preparation, determination of elemental concentrations and stable isotope ratios were carried out using an Atomic Absorption Spectrometer (AAS) and Isotope Ratio Mass Spectrometry (IRMS) respectively. Trace elements (Mn, Fe, Cu, Zn), cations (Na, K, Ca, Mg) and stable isotope (δ13C and δ15N) ratios were found to follow different patterns along the inundation gradient in the leaves of the sampled plant species. Elemental concentrations of Na, K, Fe, Cu, Zn and Mn along with stable istopic ratio differed significantly within similar and different mangrove species. The results of Rhizophora apiculata elemental concentrations and stable isotope ratios can potentially serve as a baseline to further the research in the MMFR compartments.

Impacts of increasing salinity and inundation on rates and pathways of organic carbon mineralization in tidal wetlands: a review

To improve our understanding of the carbon cycling response to imminent sea-level rise and saltwater intrusions, we review the existing literature on the likely effects of the increasing salinity and inundation on organic carbon mineralization in tidal wetlands. Enhanced salinity and inundation will reduce the pool of the organic carbon substrate, but may expand that of microbes with strong capacities for carbon metabolism. Sulfate availability increases with the increasing salinity, while availability of other electron acceptors, e.g., oxygen, nitrate, ferric oxides, and carbon dioxide, could transiently increase but would ultimately fall with the increasing salinity and inundation. The changing electron acceptor pattern may result in microbial sulfate reduction predominating over other carbon mineralization pathways. Data derived from natural salinity and inundation gradients suggest clear negative effects of salinity and inundation on production rates or emission fluxes of carbon dioxide and methane. However, results for brackish wetlands are conflicting, probably due to their unique geographic location. Salinity and inundation exert their influence on organic carbon mineralization through sulfate enrichment, elevating ionic and osmotic stress and decreasing oxygen concentrations and redox conditions, among other biogeochemical changes. Future studies should address the combined effects of salinity and inundation on carbon biogeochemistry in low-level salinity tidal wetlands.

Experimental salt marsh islands: A model system for novel metacommunity experiments

Shallow tidal coasts are characterised by shifting tidal flats and emerging or eroding islands above the high tide line. Salt marsh vegetation colonising new habitats distant from existing marshes are an ideal model to investigate metacommunity theory. We installed a set of 12 experimental salt marsh islands made from metal cages on a tidal flat in the German Wadden Sea to study the assembly of salt marsh communities in a metacommunity context. Experimental plots at the same elevation were established within the adjacent salt marsh on the island of Spiekeroog. For both, experimental islands and salt marsh enclosed plots, the same three elevational levels were realised while creating bare patches open for colonisation and vegetated patches with a defined transplanted community. One year into the experiment, the bare islands were colonised by plant species with high fecundity although with a lower frequency compared to the salt marsh enclosed bare plots. Initial plant community variations due to species sorting along the inundation gradient were evident in the transplanted vegetation. Competitive exclusion was not observed and is only expected to unfold in the coming years. Our study highlights that spatially and temporally explicit metacommunity dynamics should be considered in salt marsh plant community assembly and disassembly.

Changes in Community-Level Riparian Plant Traits over Inundation Gradients, Colorado River, Grand Canyon

Comparisons of community-level functional traits across environmental gradients have potential for identifying links among plant characteristics, adaptations to stress and disturbance, and community assembly. We investigated community-level variation in specific leaf area (SLA), plant mature height, seed mass, stem specific gravity (SSG), relative cover of C4 species, and total plant cover over hydrologic zones and gradients in years 2013 and 2014 in the riparian plant community along the Colorado River in the Grand Canyon. Vegetation cover was lowest in the frequently inundated active channel zone, indicating constraints on plant establishment and production by flood disturbance and anaerobic stress. Changes in trait values over hydrologic zones and inundation gradients indicate that frequently inundated plots exhibit a community-level ruderal strategy with adaptation to submergence (high SLA and low SSG, height, seed mass, C4 relative cover), whereas less frequently inundated plots exhibit adaptation to drought and infrequent flood disturbance (low SLA and high SSG, height, seed mass, C4 relative cover). Variation in traits not associated with inundation suggests niche differentiation and multiple modes of community assembly. The results enhance understanding of future responses of riparian communities of the Grand Canyon to anticipated drying and changes in hydrologic regime.

Frequent inundation helps counteract land use impacts on wetland propagule banks

AbstractQuestionHow do contrasting influences of inundation and historical land uses affect restoration of soil propagule bank composition in floodplain wetlands?LocationNorthern Nature Reserve (large ephemeral floodplain), Macquarie Marshes, New South Wales, Australia.MethodsWe conducted germination assays on soil samples collected from fields with different land‐use histories, stratified along an inundation gradient. We used GLM to determine whether native and exotic species richness and abundance varied along gradients of inundation and land use.ResultsSpecies richness and plant abundance in soil propagule banks were positively related to inundation and negatively related to intense historic land use. The abundance of native species was significantly higher in more frequently inundated areas. Abundances of exotic and ruderal species were higher in areas of intense prior land use. Overall species richness was generally similar across land‐use histories.ConclusionsLand‐use legacies compromised the ability of propagule banks to rejuvenate native vegetation in this floodplain wetland, especially in less frequently flooded parts of the floodplain, which harboured more ruderal and exotic species. Negative effects of prior land use may be alleviated by increased inundation. Native soil propagule banks were remarkably intact, providing a reservoir for restoration of wetland vegetation, even in soils highly disturbed by up to 20 yr of agricultural cropping. With appropriate inundation, soil propagule banks in less degraded areas of the Macquarie Marshes can provide diverse mixtures of desired species in high abundance but, in highly degraded areas, full restoration may be delayed.